lumia 720 have connect fastest 3g net speed is so high but not download any video and not even browse even ...anyne can tell me about this why this problem have

I think that might be a network issue. Just adjust your network settings. From 3g, choose 2g as highest speed & again change it to 3g as highest speed. That should do it. If this doesn't work, try choosing a manual network connection & soft reset your phone. This would surely do it.